JP7061622B2 - Methods and systems for removing harmful compounds from flue gas using fabric filter bags with SCR catalysts - Google Patents

Description

The present invention relates to methods and systems for reducing particulate matter and nitrogen oxides (NOx) emitted from off-gas and flue gas (fuel exhaust). In particular, the methods and systems of the invention provide improved NOx reduction at low gas temperatures.

A large number of boilers, gas oil-fired generators or cement kilns, biofuel combustion plants, and flue gas from various combustion facilities, such as solid or liquid field power plants. Contains environmentally problematic or even toxic compounds. These include particulate matter and NOx.

The use of particle filters and catalytic purification of flue gas is generally beneficial to the environment as it reduces the amount of particulate matter and NOx. In most areas, the reduction of NOx in flue gas is required by law.

Fabric filter bags are widely used in many industries for the removal of particulate matter from flue gases and process gases. These are the most efficient types of dust collectors.

As mentioned above, off-gas and flue gases very often contain multiple pollutants, especially nitrogen oxides, in concentrations that should be removed or reduced according to national legislation. Several conventional methods are available for this purpose. In any case, additional units need to be installed and operated upstream / downstream of the fabric filter bag.

The assignor of US Provisional Patent Application No. 2017/0080387 (Patent Document 1), which is incorporated herein by reference in its entirety, discloses a filter bag catalyzed by an SCR active catalyst. When off-gas or flue gas passes through these filter bags, the filter bags reduce or remove concentrated NOx and particulate matter.

Due to the composition of the fibers that make up the filter bag, they must be operated at temperatures below 250 ° C.

In the selective catalytic reduction (SCR) of NOx, the nitrogen oxide compound is selectively reduced to harmless nitrogen and water by reacting with a reducing agent such as ammonia on the catalyst.

The low gas temperature required by the composition of the filter bag faces problems in the removal of NOx by SCR.

The problem with known SCR catalysts is their relatively low efficiency at gas temperatures below 250 ° C.

US Provisional Patent Application No. 2017/0080387

The present invention is based on producing NO ₂ outside the flue gas duct and injecting the prepared NO ₂ into the flue gas in an amount that promotes the so-called "Fast" SCR reaction. NO ₂ can be produced from NH ₃ by the oxidation of NH ₃ to NO on the noble metal-containing catalyst in the first step and then to NO ₂ of NO in the second step.

It is known that "Fast" SCR reaction: 2NH ₃ + NO + NO ₂ → 2N ₂ + 3H ₂ O can significantly promote the SCR reaction and significantly increase the low temperature activity with a medium molar amount of NO and NO ₂ in the flue gas. be.

Therefore, in the first aspect, the present invention is a method for removing nitrogen oxides from the flue gas of a combustion facility.
In the presence of either ammonia itself added to the flue gas or in the form of its precursor, the flue gas is catalyzed by a catalyst for the selective reduction of nitrogen oxides into one or more fabric filter bags. Steps to pass;
Injecting nitrogen dioxide-containing exhaust gas into flue gas upstream of one or more fabric filter bags;
A step of catalytically oxidizing ammonia or its precursor in the presence of an oxidation catalyst in an oxygen-containing atmosphere to produce an exhaust gas containing nitric oxide and oxygen.
Provided is a method comprising a step of obtaining a nitrogen dioxide-containing exhaust gas by cooling the exhaust gas to an ambient temperature and oxidizing the nitric oxide in the cooled exhaust gas to the nitrogen dioxide-containing exhaust gas.

A problem with known methods and systems is the relatively low efficiency of SCR catalysts at flue gas temperatures below 250 ° C. as described herein. This problem is solved by the present invention involving injecting NO ₂ into the flue gas at low temperatures to facilitate the "Fast" SCR reaction. This reaction is involved in promoting SCR at low temperatures.

When the flue gas temperature is above 250 ° C, the SCR catalyst has sufficient efficiency, and when the gas temperature reaches 250 ° C, the injection of NO ₂ into the flue gas can be interrupted.

Oxidation of ammonia to NO outside the flue gas duct is usually in the presence of an oxygen-containing atmosphere at a reaction temperature of 250-800 ° C., typically platinum or platinum as a trace component with other noble metals. It is carried out in a reactor having a noble metal catalyst such as an alloy.

To reach the required reaction temperature, the oxidation reactor can be heated, for example, by energization heating or induction heating.

In one embodiment, the oxygen-containing atmosphere comprises a hot recirculated gas, which in turn is part of the heating load of the oxidation reactor.

Equilibrium reaction

を進めてＮＯ_２を生成するために、ＮＯ含有排ガスを周囲温度まで冷却することにより、貴金属含有触媒との接触によるＮＨ_３の酸化により第１のステップにおいてＮＨ_３から生成されたＮＯをその後酸化して、第１のステップからのＮＯ含有排ガス中のＮＯ_２にする。

By cooling the NO-containing exhaust gas to the ambient temperature in order to proceed to generate NO ₂ , the NO produced from NH ₃ is subsequently oxidized in the first step by the oxidation of NH ₃ by contact with the noble metal-containing catalyst. Then, it becomes NO ₂ in the NO-containing exhaust gas from the first step.

As used herein, the term "ambient temperature" is used to mean any temperature predominant around a combustion facility that uses the methods and systems of the invention. Typically, the ambient temperature is −20 ° C. to 40 ° C.

Cooling and oxidation of the NO-containing exhaust gas can be performed in an aging reactor sized so that the residence time of the gas is about 1 minute or more.

In one embodiment, the oxidation reaction is carried out in the presence of a catalyst that promotes the oxidation of NO to NO ₂ . These catalysts are known in the art and include Pt on TiO ₂ , Pt on SiO ₂ , and activated carbon or Pt and / or Pd on alumina.

As mentioned above, the desired Fast SCR reaction requires equal amounts of NO and NO ₂ . Therefore, the amount of NO ₂ injected into the flue gas at a temperature below 250 ° C. is such that 45 to 55% by volume of the nitrogen oxide content in the flue gas is NO ₂ at the inlet of the SCR catalyst unit. Is controlled by.

In another aspect, the invention provides a system for use in the method according to the invention.

The system is a filter bag house with one or more fabric filter bags catalyzed by a catalyst for selective reduction of nitrogen oxides in a flue gas duct;
A means of injecting ammonia or urea solution into a flue gas duct upstream of one or more fabric filter bags or filter bag houses;
Injection means for injecting nitrogen dioxide-containing exhaust gas upstream of one or more fabric filter bags or filter bag houses; as well as outside the flue gas duct.
The nitric oxide-containing exhaust gas recovered from the ammonia oxidation catalyst, which is connected to the ammonia oxidation catalyst unit and the injection means for injecting the nitrogen dioxide-containing exhaust gas at its outlet end, is cooled and oxidized to the nitrogen dioxide-containing exhaust gas. Including means for.

As described above, the oxidation reaction of NO to NO ₂ requires a residence time of NO-containing gas for at least 1 minute. Typically 1-2 minutes.

This is achieved in either gas-cooled or water-cooled heat exchangers, or in cooling and oxidizing means such as swirl tubes that, when molded, have a length that allows the gas to pass through the tube with the desired residence time. Can be done.

In another embodiment, the means for cooling and oxidizing the nitric oxide-containing exhaust gas comprises an oxidation catalyst that promotes the oxidation of NO to NO ₂ .

Claims (9)

A method of removing nitrogen oxides from the flue gas of combustion equipment.
The step of passing the flue gas through one or more fabric filter bags with a catalyst for the selective reduction of nitrogen oxides in the presence of either ammonia itself added to the flue gas or in the form of its precursors. ;
Injecting nitrogen dioxide-containing exhaust gas into flue gas upstream of one or more fabric filter bags;
A step of catalytically oxidizing ammonia or its precursor in the presence of an oxidation catalyst in an oxygen-containing atmosphere to produce an exhaust gas containing nitric oxide and oxygen.
A step of obtaining an exhaust gas containing nitrogen dioxide by cooling the exhaust gas containing nitric oxide and oxygen to an ambient temperature and oxidizing the nitrogen monoxide in the cooled exhaust gas to the exhaust gas containing nitrogen dioxide. Including the method. The method of claim 1, wherein the oxygen-containing atmosphere comprises flue gas. The method according to claim 1, wherein the oxygen -containing atmosphere is ambient air.
Claims 1-3, wherein the nitrogen dioxide-containing exhaust gas is injected into the flue gas at the inlet of the catalyst for selective reduction of the nitrogen oxide in an amount such that 45 to 55% by volume of the nitrogen oxide becomes nitrogen dioxide. The method described in any one of. The method according to any one of claims 1 to 4, wherein the oxidation of nitric oxide in the cooled exhaust gas to the nitrogen dioxide-containing exhaust gas is performed in the presence of an oxidation catalyst. A system for use in the method according to any one of claims 1 to 5.
A filter bag house with one or more fabric filter bags with a catalyst for the selective reduction of nitrogen oxides in the flue gas duct;
A means of injecting ammonia or urea solution into a flue gas duct upstream of one or more fabric filter bags or filter bag houses;
Injection means for injecting nitrogen dioxide-containing exhaust gas upstream of one or more fabric filter bags or filter bag houses; as well as outside the flue gas duct.
The nitric oxide-containing exhaust gas recovered from the ammonia oxidation catalyst unit , which is connected to the ammonia oxidation catalyst unit and the injection means for injecting the nitrogen dioxide-containing exhaust gas at its outlet end, is cooled and oxidized into the nitrogen dioxide-containing exhaust gas. A system that includes means to do. The system according to claim 6, wherein the means for cooling and oxidizing the nitric oxide-containing exhaust gas is a heat exchanger . The system according to claim 6, wherein the means for cooling and oxidizing the nitric oxide-containing exhaust gas is a spiral tube . The system according to any one of claims 6 to 8, wherein the means for cooling and oxidizing the nitric oxide-containing exhaust gas includes an oxidation catalyst.